The present invention relates to a control technique for a radiotelephone communication system, and more particularly, to a control technique for a wireless communication system.
Continuing growth in telecommunications is placing increasing stress on the capacity of cellular systems. The limited frequency spectrum made available for cellular communications demands cellular systems having increased network capacity and adaptability to various communications traffic situations. Although the introduction of digital cellular systems has increased potential system capacity, these increases alone may be insufficient to satisfy added demand for capacity and radio coverage. Other measures to increase system capacity, such as decreasing the size of cells in metropolitan areas, may be necessary to meet growing demand.
Interference between communications in cells located near one another creates additional problems, particularly when relatively small cells are utilized. Thus, techniques are necessary for minimizing interference between cells. One known technique is to group cells into "clusters". Within individual clusters, communications frequencies are allocated to particular cells in a manner which attempts to maximize the uniform distance between cells in different clusters which use the same communication frequencies. This distance may be termed the "frequency reuse" distance. As this distances increases, the interference between a cell using a communication frequency and a distant cell using that same frequency is reduced.
Radio base stations are often located near the center of each cell to provide radio coverage throughout the area of the cell. Alternatively, a radio base station may be located near the center of three adjacent "sector cells" to cover those cells. The choice between a sectorized and non-sectorized system is based on various economical considerations such as equipment costs for each base station.
Localized microcells and picocells may be established within overlying macrocells to handle areas with relatively dense concentrations of mobile users, sometimes referred to as "hot spots". Typically, microcells may be established for thoroughfares such as crossroads or streets, and a series of microcells may provide coverage of major traffic arteries such as highways. Microcells may also be assigned to large buildings, airports, and shopping malls. Picocells are similar to microcells, but normally cover an office corridor or a floor of a high-rise building. The term "microcells" is used in this application to denote both microcells and picocells, and the term "macrocells" is used to denote the outermost layer of a cellular structure. An "umbrella cell" can be a macrocell or a microcell as long as there is a cell underlying the umbrella cell. Microcells allow additional communication channels to be located in the vicinity of actual need, thereby increasing cell capacity while maintaining low levels of interference.
The design of future cellular systems will likely incorporate macrocells, indoor microcells, outdoor microcells, public microcells, and restricted microcells. Macrocell umbrella sites typically cover radii in excess of 1 kilometer and serve rapidly moving users, for example people in automobiles. Microcell sites are usually low power, small radio base stations, which primarily handle slow moving users such as pedestrians. Each microcell site can be viewed as an extended base station which" is connected to a macrocell site through digital radio transmission or optical fibers.
In designing a microcell cluster, it is necessary to allocate spectrum to the microcells. This can be done in several ways; for example, microcells can reuse spectrum from distant macrocells; a portion of the available spectrum may be dedicated for microcell use only; or a microcell can borrow spectrum from an umbrella macrocell.
In dedicating spectrum to the microcells, a portion of the available spectrum is reserved strictly for the microcells and unavailable to macrocells. Borrowing spectrum involves taking frequencies available to a covering macrocell for microcell use.
Each of these channel allocation methods has accompanying advantages and drawbacks. Reusing channels from distant macrocells causes little reduction in capacity of the macrocell structure. However, reuse is not always feasible because of co-channel interference between the microcells and macrocells.
By dedicating spectrum to the microcell, interference between cell layers (microcell and macrocell) is reduced because any co-channel interference is between microcells, not between macrocells and microcells. When dedicating spectrum to a microcell, that spectrum is taken from the entire macrocell system in a certain area, e.g., a city. Thus, that spectrum is not available for macrocell use. As a result, in an area containing only a few microcells, capacity is adversely affected because the microcells cover only a small portion of the area in the macrocell area while the macrocell, with a reduced amount of spectrum available, must cover a substantial area. Nevertheless, as the number of microcells increases and the area covered by only the macrocell decreases, capacity problems associated with dedicating spectrum may be reduced and a total net gain in overall system capacity can be achieved without introducing blocking in the macrocells.
Borrowing channels from an umbrella macrocell, like reuse, presents potential co-channel interference between microcells and macrocells. Also, capacity may be adversely affected because efficient spectrum allocation is often impossible. For example, it may be difficult to address all the hot spots in a cell simultaneously when borrowing or dedicating spectrum. An advantage of borrowing spectrum is that the entire macrocell system is not affected, unlike dedicating spectrum, because only spectrum allocated to a covering macrocell is borrowed and not spectrum from the entire system. Thus, other macrocells can use the same spectrum which is being borrowed by a microcell from its covering macrocell.
Further, in cluster design, allocated spectrum must be distributed to individual microcell sites. Known methods employed for spectrum allocation include fixed frequency planning, dynamic channel allocation (DCA), and adaptive channel allocation (ACA). Further, a control channel management technique must be selected. One possibility includes having each cell or sector in a sectorized system use a unique control channel until frequency reuse is feasible from an interference point of view.
With the introduction of microcells, radio network planning may increase in complexity. The planning process is largely dependent upon the structure of the microcells. For example, the sizes of streets, shopping malls, and buildings are key design criteria. Microcells suffer from a series of problems including an increased sensitivity to traffic variations, interference between microcells, and difficulty in anticipating traffic intensities. Even if a fixed radiotelephone communication system could be successfully planned, a change in system parameters such as adding a new base station to accommodate increased traffic demand may require replanning the entire system. For these reasons, the introduction of microcells benefits from a system in which channel assignment is adaptive both to traffic conditions and to interference conditions.
One of the main concerns associated with microcells is the minimization of frequency planning in FDMA and TDMA systems or power planning in a CDMA system. Radio propagation characteristics which are dependent on environmental considerations (e.g., terrain and land surface irregularities) and interference are difficult to predict in a microcellular environment, thereby making frequency or power planning extremely difficult if not impossible. One solution is to use an ACA scheme which does not require a fixed frequency plan. According to one implementation of this method, each cell site can use any channel in the system when assigning a radio channel to a call. Channels are allocated to calls in real time depending on the existing traffic situation and the existing interference situation. Such a system, however, may be expensive since more channel units on the average must be installed.
Several advantages are realized with ACA. There is almost no trunking efficiency loss since each cell can use any channel. Thus, it is possible to employ cells with very few channels without losing network efficiency. Further, channel reuse is governed by average interference conditions as opposed to the worst-case scenario.
Several ACA schemes attempt to improve traffic capacity and avoid the need for frequency planning. While some systems have been moderately effective in accomplishing these goals, it has been very difficult to fully achieve both goals in a system which has preassigned control channels, i.e., a system having specified frequencies on which a mobile station may expect a control channel (a 30 KHz RF channel which contains control signals). Systems having preassigned control channels include AMPS (Advanced Mobile Phone Service System), IS-54 (Revision B) and TACS (Total Access Communication System). In such systems, frequency planning is still needed for control channels. However, frequency planning can be avoided and traffic capacity improved by eliminating the need to plan a number of voice channels on each site in an area where traffic channels are expected to be non-uniformly distributed.
In many systems, microcells may be control channel limited rather than voice channel limited for capacity. For example, in a 7/21 cell plan commonly used in the AMPS system, frequencies are assigned to ensure that cells using the same frequency are separated by a reuse distance which maintains interference below certain predetermined criteria, e.g., carrier to interference (C/I) ratio. In a 7/21 macro system, the spectrum of each cluster is divided into 21 frequency groups, each group containing a number of channels distinct from each other group. There are seven sites each with three sectors in a 7/21 plan. Each sector is assigned to one frequency group. In the area outside the seven sites, the frequency is reused, i.e., the same frequencies may be used again in adjacent clusters.
In a typical 7/21 cellular system, each base station represents a site and each cell represents a sector. A microcell located within an umbrella cell is unable to use the same frequency as the umbrella cell unless there is a very high penetration loss to an area inside the microcell. As a result, the spectrum from the microcell must be reused from a distant macrocell, borrowed from the umbrella cell, or dedicated from the spectrum available to the cellular system.
When reusing spectrum from a distant macrocell, the number of macrocells from which frequency may be reused is based on the radio propagation environment (i.e., the terrain between the cells) and the interference criteria. The reuse distance is designed so that co-channel interference is limited to acceptable ranges. For example, in the AMPS system, the desired signal preferably ranges from ten to one hundred times greater than the interfering signal.
In addition to reusing spectrum in a microcell from a distant macrocell, assuming no spectrum has been dedicated, generally referred to as "reuse assignment", two other reuse processes exist, one for the entire macro system and one where the assigned spectrum is reused inside the area (cluster) of the micro system. If it is possible to assign spectrum of the macrocell to the microcell from only two distant macrocells as a result of interference, the microcell area will have only two control channels. The number of voice channels depends on how many voice channels are assigned to these particular macrocells. In systems currently employed in the United States (the "cellular band" with two operators) there are approximately 400 channels available per system. The average macrocell (sector) in a 7/21 plan has approximately eighteen voice channels. Therefore, when beginning the planning process for the microcell area, there are thirty-six voice channels and two control channels.
An omnicell system uses one base station for each cell. In an omnicell system, for example, a 12/12 system, the interference distribution may differ from that of the 7/21 system. In certain operating environments in the microcell, it may be possible to have for example a twelve-site reuse plan since the same quality aspects must be achieved. Thirty-six voice channels with fixed frequency planning on a twelve-site reuse plan may result in three channels per site. The thirty-six voice channels can be allocated to another set of sites adjacent to the first twelve sites, but there are only two control channels. A two-site reuse plan in a two dimensional cluster area results where the same frequency is used in adjacent cells. In this example, only two cells can be installed. Thus, the quality criteria for the above example which requires a minimum twelve-cell reuse plan cannot be satisfied to achieve sufficient radio link quality. Accordingly, such microcell channel systems may be control channel limited, i.e., the intended cell cluster cannot operate properly because the control channels will be jammed with co-channel interference.
A potential solution to this problem is to increase the size of microcells by, for example, increasing the power, so that two microcells provide sufficient radio coverage. According to this solution, spectrum reuse is no longer required assuming radio coverage, and not capacity, is the primary concern in planning the microcell area. However, this may not be possible because interference between the microcells and macrocells may exceed acceptable levels. To overcome this problem, two microcells may be used in conjunction with many antennas in a distributed antenna system. This allows the area of coverage to extend over the microcell cluster area without requiring a high relative transmission power because the mobiles at the fringe of a cell will be closer to one of the antennas in comparison to a single antenna system.
This type of implementation has limitations. Additional RF cabling is necessary which results in attenuated signals. If the microcell system area is large, there may be insufficient power left at the remote antenna. In turn, this might require a costly high power amplifier in the base station. Moreover, when high power base station transmitters compensate for cable loss, mobile stations also must transmit with high power to overcome the cable loss. Therefore, there is little incentive to design the power amplifier in the base station to permit transmission higher than the power transmitted from the mobile (AMPS hand-held 0.6 watt) in the uplink, taking into account gain (4-7 dB) attributable to antenna diversity at a typical base station. One drawback with such systems is that mobiles are forced to transmit with relatively more power, thereby shortening the battery life in a mobile station and undermining one of the purposes for introducing microcells. More importantly, high powered mobiles are more likely to interfere with macrocells using the same spectrum unless the microcells use a dedicated spectrum.
Alternatively, a system employing distributed power amplifiers in conjunction with optical fibers may be used. According to such a system, a remote controller would transmit a light signal which would be amplified. The signal would be received locally where it is converted back to a radio signal. Losses associated with cabling in a system employing optical fibers may be minimized because the signals do not need to be amplified very often and in most typical scenarios not at all. Further, an optical fiber system provides added flexibility and can be easily installed. However, it is expensive to implement a system having optical interfaces.
When planning an antenna system, allocating spectrum for a microcell cluster, and selecting a power level for microcell transmitting power, several concerns must be addressed. Sufficient radio coverage, e.g., 98%, must be provided within the microcell area. Also, if the spectrum allocated to the microcell cluster has been reused from a distant macrocell, the power level of the microcells must be low enough to avoid interference with the distant macrocell from which the spectrum was reused. Further, the power of the control channel in the microcell may have to be stronger than the power of the covering umbrella macrocell control channel if the mobile is to lock on to the microcell. In sum, the aim of such a system is to assign as many mobiles as possible to microcell control channels by maintaining those control channels stronger than the control channels of the umbrella macrocell in the intended microcell area while transmitting with a sufficiently low power to avoid interference with the distant macrocell.
Power or interference limitations can result in a voice channel limited system where some of the mobiles in the microcells will receive a stronger signal from an overlying macrocell. The number of mobiles receiving a stronger signal from an overlying macrocell will increase as the distance between the umbrella cell and the microcell is shorter. Consequently, capacity might not increase since mobiles are locked-on to the macrocell. Moreover, if mobile transmitting power requirements increase, the battery life of the current portables would correspondingly decrease to maintain the equivalent level of performance. Further, blocking and intermodulation distortion may arise with mobiles located inside the microcell area, close to the microcell base station, but power controlled by a macrocell. The mobiles are power controlled by the umbrella macrocell and require more power to communicate with the umbrella macrocell than the microcell.